1. Field of the Invention
This invention relates to a process for producing printed circuit boards.
2. Description of the Prior Art
Conventional methods of printed circuit board manufacturing consist of essentially two techniques: the subtractive method and the additive method. In the subtractive method a substrate having a relatively thick copper laminate is photoresist coated and exposed through a mask according to the desired conductor configuration. After exposure, the photoresist is removed from the unexposed areas and then the circuit board is etched, removing the copper from the area not covered by the photoresist. The remaining resist is subsequently removed by means of a solvent, so that only the printed circuit remains on the insulating substrate.
This subtractive method has the disadvantage that a relatively thick copper laminate is applied and then most of this laminate has to be removed by etching. As a result, this method is uneconomical, as much of the copper is wasted. Additionally, the geometry of the conductor structure is limited because the etchant is capable of undercutting the desired copper conductors as well as removing the unwanted copper.
In the additive method, a thin copper foil of about 5 microns thickness is laminated to an insulating substrate. Subsequently, a photoresist film is laminated to the copper foil and exposed and developed as in the subtractive process. The photoresist film is then removed from those areas where the copper conductors are to be generated and in an electroless copper bath, additional copper is deposited on the bared copper conductors. The photoresist mask is then removed.
The common features of the subtractive and additive processes are: (1) they both use negative photoresists which are applied either as a liquid or as a resist foil, and (2) they both result in a conductor structure that is raised above the level of the insulating substrate.
It is desirable, however, to utilize positive photoresist where possible, and under some circumstances, it is also desirable to have a conductor structure that is embedded in the insulating substrate and less vulnerable to environmental factors, such as rubbing, scratching, etc.
A process is known for producing circuit boards with planar conductor structures embedded therein, but this prior art process is a subtractive one using negative photoresist materials.
Therefore, it is the object of this invention to provide an improved process for producing printed circuit boards having conductor structures embedded in an insulating substrate wherein positive photoresist materials are advantageously used and a planar surface is obtained.
It is a further object of the invention to provide a process wherein conductor structures are present on both the front and back sides of a insulating substrate and plated through holes connect the two conductor structures.